I frequently work on the writing of others, chiefly foreign speakers for whom English is a second language. Proofreading is the lightest correcting I do, revisions involve the most work. People argue greatly over what these terms mean. Here are some standard definitions along with my thoughts.
Proofreading: Careful reading and rereading of a yet to be finally-printed document, to detect any errors in spelling, punctuation, or grammar.
Proofreading may involve nothing more than adding or removing commas and apostrophes, correcting misspellings, and making consistent the use of certain words and phrases. With proofreading, the document stays strongly with the writer’s voice.
Editing: Arranging, revising, and preparing a written, audio, or video material for final production, usually by a party other than the creator of the material. The objectives of editing include (1) detection and removal of factual, grammatical, and typographical errors, (2) clarification of obscure passages, (3) elimination of parts not suitable for the targeted audience, and (4) proper sequencing to achieve a smooth, unbroken flow of narrative.
Editing demands more work. At this point, the editor becomes a shadow collaborator, occasionally taking command of the piece. If done well, editing retains the voice of the writer, consistent throughout.
Revising: In revising you can change the way you present the material. Revising is more time consuming compared to editing because it involves more critical thinking.
Revising is the most difficult task. When a document has multiple revisions, it becomes extremely hard to make the work consistent, it becomes patchy, as the words of the writer go one way, the words of the editor the other. The editor is now a full partner in the piece. Recommendations for new content or material may be made. Openings, summaries and conclusions may be debated. Although deadlines rarely permit a complete revision of the writing, a full rewriting is sometimes the only way to make the document flow smoothly and with sense.